Yes it can be, with the viewport meta tag. I've also seen some web apps, mostly maps, whose CSS or JS tries to resist browser zoom in ways that break the layout

Yes it can be, with the viewport meta tag. I've also seen some web apps, mostly maps, whose CSS or JS tries to resist browser zoom in ways that break the layout or make them unusable. Plus, overriding the zoom gestures makes it really hard in mobile browsers to access the actual browser zoom, although I blame that on browsers.